All good questions:
1. No not possible as far as I know. So you should create a Global Template and each activity should be based on this template
2. I don't'know the REST API by heart. But this must be possible (since you can create an activity from the Notes Activity Plugin, I assume the REST API must provide this)
3. Again, it is possible with the REST API to extract data from an activity.
You can find info over the API here:
Hope this helps